1. Field of the Invention
The present invention relates to a semiconductor device including a thin-film transistor used as a switching element, a method for manufacturing the same, and a radiation detector. More particularly, the invention relates to a semiconductor device for photoelectric conversion having pixels, each including a photoelectric transducer and a thin-film transistor, a method for manufacturing the same, and a radiation detector.
2. Description of the Related Art
Recently, modules using thin-film transistors are being used in various fields. Such modules include, for example, liquid-crystal display devices or organic EL (electroluminescent) displays, each using thin-film transistors as switching elements on an insulating surface of a substrate, large flat-panel sensors, each using thin-film transistors as switching elements on an insulating surface of a substrate, and the like. A large flat-panel sensor is used as a detector for radiation, such as X-rays, by forming a layer of a substance called a scintillator or a phosphor above the sensor.
Although the size of the substrate is increasing, intension to realize a small-size and high-precision semiconductor-device module using thin-film transistors is very strong, since such a module is used as a display device for a portable terminal, a cellular phone or the like. In such circumstances, in order to improve the performance of a thin-film transistor, it is desired to improve the transfer efficiency of the thin-film transistor and also improve the numerical aperture of the corresponding pixel by reducing the size of the thin-film transistor. The situation is the same for a flat-panel sensor, in which it is also necessary to maintain the sensitivity of the sensor while achieving high-speed driving.
At present, bottom-gate-type thin-film transistors in each of which a gate electrode is formed on an insulating substrate and a semiconductor layer is formed on the gate electrode is mostly used as thin-film transistors. The bottom-gate-type thin-film transistors are grossly classified into two types.
One type comprises thin-film transistors called a gap-etching type or a channel-etching type, as shown in FIG. 9. In this type, after forming a gate electrode 2 on an insulating substrate 1, an insulating film 3, a semiconductor layer 4 and a doped semiconductor layer 5 are consecutively formed by CVD (chemical vapor deposition), and a thin-film transistor is formed by etching the doped semiconductor layer 5 at a gap portion of the thin-film transistor. In this gap-etching-type thin-film transistor, since the semiconductor layer 4 is formed thin, it is necessary to improve the distribution of etching at gap etching and make the thickness of the semiconductor layer 4 uniform during film formation.
Another type comprises thin-film transistors called, for example, a etching-stopper type or a channel-passivation type, as shown in FIG. 10. In this type, after forming a gate electrode 2 on an insulating substrate 1, an insulating film 3, a semiconductor layer 4, and a channel-protection film 8 comprising, for example, an insulating film, are consecutively formed by CVD. Then, the channel-protection film 8 is etched except for a portion corresponding to a gap portion of the thin-film transistor, followed by formation of a doped semiconductor layer 5. Then, a thin-film transistor is formed by etching the doped semiconductor layer 5 at a gap portion of the thin-film transistor. In this etching-stopper-type thin-film transistor, although a semiconductor layer can be formed independent of the distribution of etching during gap etching, control when etching the insulating film 8 is important. A high-speed thin-film transistor is provided by stabilizing the etching rate, improvement of the distribution of etching, and the like.
In the etching-stopper-type thin-film transistor using an insulating film comprising, for example, a silicon-nitride film or the like, it is pointed out that, although it is possible to provide a high-performance thin-film transistor by forming a thin semiconductor layer, the number of processes increases, resulting in a large process time.
In the gap-etching-type thin-film transistor, it is pointed out that, although the manufacturing process is relatively simple, it is difficult to form a thin semiconductor film because a dopant is unintentionally injected to a predetermined depth from the surface of the semiconductor layer while the doped semiconductor layer is formed. If the thickness of the semiconductor layer is large, the operation of the thin-film transistor is slow.
It is considered that in any type of thin-film transistor, it is difficult to sufficiently improve the quality of a semiconductor film, serving as a channel, if the thickness of the semiconductor film is very thin, in consideration of the manufacturing process.
In any case, a thin-film transistor capable of performing a high-speed operation using a good-quality thin film as a semiconductor layer, serving as a channel, is being desired.
It is an object of the present invention to provide a semiconductor device having thin-film transistors capable of performing a high-speed operation, a method for manufacturing the same, and a radiation detector using the semiconductor device.
It is another object of the present invention to provide a semiconductor device having thin-film transistors having excellent transfer efficiency, a method for manufacturing the same, and a radiation detector using the semiconductor device.
It is still another object of the present invention to provide a semiconductor device having inexpensive thin-film transistors which can prevent a decrease in the sensitivity of photoelectric transducers when integrating the thin-film transistors with the photoelectric transducers, a method for manufacturing the same, and a radiation detector using the semiconductor device.
According to one aspect of the present invention, in a semiconductor device including bottom-gate-type thin-film transistors each of which includes a gate electrode provided on an insulating surface of a substrate, a semiconductor layer provided on the gate electrode via a gate insulating layer, a pair of doped semiconductor layers adjacent to the semiconductor layer, and source and drain electrodes consisting of a pair of conductors adjacent to corresponding ones of the pair of doped semiconductor layers, a thickness of portions of the semiconductor layer below the source and drain electrodes is smaller than a thickness of a portion of the semiconductor layer at a gap portion between the source and drain electrodes.
In this invention, the thickness of the portions of the semiconductor layer below the source and drain electrodes may be within a range of 30 nm-300 nm, and the thickness of the portion of the semiconductor layer at the gap portion may be within a range of 60 nm-1,500 nm.
The thickness of the portions of the semiconductor layer below the source and drain electrodes may be 0 nm.
The surface of the gap portion may be covered with a protective film covering the source and drain electrodes. The surface of the gap portion may be covered with a channel-protection film, and end portions of the channel-protection film may be covered with the source and drain electrodes.
The doped semiconductor layer may be formed on the semiconductor layer which has been thinned by etching.
In this invention, photoelectric transducers may also be provided on the insulating surface of the substrate.
Each of the photoelectric transducers may include a semiconductor layer made of a material which is the same as a material for the semiconductor layer at the gap portion of the thin-film transistor and whose thickness is the same as the thickness of the semiconductor layer at the gap portion. Each of the photoelectric transducers may include a semiconductor layer made of a material which is the same as a material for the semiconductor layer at the gap portion of the thin-film transistor and whose thickness of the same as the thickness of the semiconductor layer at the gap portion, a doped semiconductor layer made of a material which is the same as a material for the doped semiconductor layer of the thin-film transistor and whose thickness is the same as a thickness of the doped semiconductor layer of the thin-film transistor, and an insulating layer made of a material which is the same as a material for the gate insulating layer of the thin-film transistor and whose thickness is the same as a thickness of the gate insulating layer.
According to another aspect of the present invention, a method for manufacturing a semiconductor device including bottom-gate-type thin-film transistors each of which includes a gate electrode provided on an insulating surface of a substrate, a semiconductor layer provided on the gate electrode via a gate insulating layer, a pair of doped semiconductor layers adjacent to the semiconductor layer, and source and drain electrodes consisting of a pair of conductors adjacent to corresponding ones of the pair of doped semiconductor layers includes the steps of forming the semiconductor layer, removing surfaces of portions of the semiconductor layer where the source and drain electrodes are to be formed, in a state in which a surface of a portion of the semiconductor layer which is to become a gap portion between the source and drain electrodes is covered with an etching mask, forming the doped semiconductor layer on the portions removed by the etching, and forming the source and drain electrodes on the doped semiconductor layers.
It is preferable that before removing the etching mask after the removing step and thereafter forming the doped semiconductor layers, at least one surface treatment selected from surface treatment by a solution containing ammonia or hydrogen chloride, and hydrogen peroxide, surface treatment by a solution containing a chelating agent, and surface treatment utilizing oxygen plasma is performed for the portions removed by the etching.
It is also preferable that before removing the etching mask after the removing step and thereafter forming the doped semiconductor layers, surface treatment for removing an organic substance is performed for the portions removed by the etching, and then surface treatment by a solution containing hydrogen fluoride is performed.
Before removing the etching mask after the removing step and thereafter forming the doped semiconductor layer, surface treatment utilizing hydrogen plasma may be performed in an apparatus for forming the doped semiconductor layer. The above-described treatment is useful for improving ohmic contact of the source and drain electrodes.
In the above-described manufacturing method, it is preferable that discharge electric power of the plasma during the surface treatment utilizing the hydrogen plasma is equal to or less than discharge electric power when forming the semiconductor layer. It is also preferable that discharge electric power during the surface treatment utilizing the hydrogen plasma is equal to or less than discharge electric power when forming the doped semiconductor layers. The above-described treatment is effective for preventing alteration of the doped semiconductor layers.
In the above-described manufacturing method, it is preferable that surfaces of portions of the semiconductor layer where the source and drain electrodes are to be formed are etched in a state in which surfaces of a semiconductor layer of a photoelectric transducer formed on the insulating surface of the substrate and the semiconductor layer of the thin-film transistor with a protective film, the protective film covering the surface of the semiconductor layer of the photoelectric transducer is etched, and the surfaces of the portions of the semiconductor layer where the source and drain electrodes are to be formed are etched deeper.
A radiation detector according to the present invention includes the above-described semiconductor device, and a controller for processing an image signal from the semiconductor device and transmitting the image signal to an external apparatus.
It is preferable that the radiation detector further includes a display device for displaying an image.
It has become clear that the following two items are required for a high-performance thin-film transistor for achieving the above-described objects.
(1) The portions of the semiconductor layer below the source and drain electrodes are made thin.
(2) The portion of the semiconductor layer at the gap portion between the source and drain electrodes is made thick.
For example, in the case of the gap-etching-type thin-film transistor, during etching of the doped semiconductor layer at the gap portion, a damaged layer having a thickness of about 20 nm-100 nm, occasionally about 20 nm-150 nm, is formed even if a surface layer of the semiconductor layer, below the doped semiconductor layer, where a dopant has been injected. As a result, an increase in the off-current supposedly due to a shift of the threshold voltage Vth of the thin-film transistor, or an increase in the on-resistance of the thin-film transistor due to the damaged layer sometimes occurs. Thus, the off-current becomes larger as the thickness of the semiconductor layer is smaller, resulting in difficulty in manufacturing a thin-film transistor having an excellent transfer efficiency.
In the case of the etching-stopper-type thin-film transistor, although the semiconductor layer can be made relatively thin, for example, the characteristics of the semiconductor layer, serving as a channel, are less sufficient as the semiconductor layer is thinner.
In the case of the gap-etching-type thin-film transistor, when the thickness of the portion of the semiconductor layer at the gap portion between the source and drain electrodes is increased, the off-current of the thin-film transistor may increase due to a decrease in the bulk resistance of the semiconductor layer. However, since it is confirmed that the off-current of the thin-film transistor is determined by leakage at interfaces of the etched portion during gap etching, the off-current is not unintentionally increased even if the thickness of the semiconductor layer at the gap portion between the source and drain electrodes is increased.
In the case of the etching-stopper-type thin-film transistor, such leakage current is much suppressed.
When using the thin-film transistor itself as the photoelectric transducer, or when integrating the thin-film transistor with the photoelectric transducer, it is desired to first form a semiconductor film having a thickness such that light can be sufficiently received, from the convenience of the manufacturing process.
In the present invention, a configuration satisfying the above-described item (2) is adopted because of the above-described reasons.
At portions near the source and drain electrodes, by relatively reducing the thickness of the semiconductor layer, it is possible to reduce the resistance of the semiconductor layer near the source and drain electrodes, thereby reducing the on-resistance of the thin-film transistor.
Accordingly, in the present invention, a configuration satisfying the above-described item (1) is adopted.